Purpose:
* manage releases through a minimum number of rake tasks
* simplify building of snapshot builds
* create staged artifacts, candidates for releases, that required no changes to become releases
* this means the snapshot release process will not involve publishing gems, therefore:
* the gem artifacts should only be published to rubygems as a final artifact, at the time of GA
Changes:
* release artifacts no longer depend on gems of core components
* all core components are used locally AS-IS to minimize code changes between snapshot, RC and GA
* `versions.yml` describes the versions of all logstash parts and package
* `rake version:set[version]` manage the yaml file and push the changes to the gemspecs/version.rb files
* `rake version:set_plugin_api[version]` manage the yaml file and push the changes to the gemspecs/version.rb files
* `rake artifact:all` generates SNAPSHOT artifacts: tar.gz, zip, rpm, deb
* `RELEASE=1 rake artifact:all` creates release candidate artifacts + 4 gems: logstash-core, logstash-core-event, logstash-core-event-java and logstash-core-plugin-api
implements #5416 and #5414Fixes#5460
from_json POC
add Event#from_json with corresponding specs
pre-inititalize error class constants
tests for Event#from_json
support array of events in from_json, upgrade to latest jackson
add test for partially invalid json array
cleaned dependencies, removed Guava code
default to java event
also compile java event
use json-unit for json result assertions
remove Java 8 specific constructs to be able to compile on Java 7
use jar-dependencies
better jar file loading
copy jar into lib after build
add post-build copy and clean of gem jar and re-indented file
fix jar destination
include generated jar in gem
remove useless ArrayList constructions
add tests for KeyNode.join method
remove useless shadow and disable unchecked warnings
update gradle dependency to 2.8
fix deprecated asserEquals usage
simplify convoluted list contruction
fix array fields nil values handling
moar nil value tests
simplify for loop
